1. Field of the Invention
The present invention relates to a temperature sensor, such as a clinical thermometer or a thermometer, for use at ordinary temperatures.
2. Description of the Related Art
As temperature sensors, generally speaking, temperature sensors using a change in resistance of a noble metal, such as platinum (Pt), or temperature sensors using oxide semiconductors of Mn or Co are known. However, since starting materials of such temperature sensors are expensive or must be calcined at high temperatures, it is difficult to reduce the cost of the temperature sensor. Accordingly, an inexpensive temperature sensor using carbon as a temperature-sensing resistance element has hitherto been proposed.
The above-described temperature sensor is formed in such a way that a paste is obtained by mixing and dispersing carbon powder extracted by calcining and carbonizing acetylene gas into an epoxy resin, this paste is coated between electrodes provided on a base film, after which the paste is printed to form a temperature-sensing resistance element. In principle, this is a temperature sensor which uses a resistance value when the temperature-sensing resistance element is brought into contact with an object to be measured, and a thermal equilibrium state is reached.
An infrared sensor has also been proposed which is arranged in such a way that coal pitch or the like is heat-treated in an inert gas at 650.degree. to 720.degree. C. in order to form carbon fibers, and the carbon fibers are stretched or crossed between the electrodes.
The above-described conventional carbon resistance temperature sensor having carbon powder mixed and dispersed into an epoxy resin makes it possible to reduce the cost in comparison with a temperature sensor using noble metals or oxide semiconductors because carbon powder which is a starting material of the temperature-sensing resistance element is inexpensive and the paste can be printed at relatively low temperatures. However, the resistivity of the carbon powder which is obtained by carbonizing acetylene gas at a high temperature of 1,800.degree. C. is extremely small, and a change in the resistance value of the temperature-sensing resistance element with respect to the change in the environment temperature is small considerably. Therefore, the range of temperature measurements is limited to extremely low temperatures of approximately 4.2 to 20 (K), and measurements are impossible at ordinary temperatures.
The conventional infrared sensor using carbon fibers has problems, for example, the shape is limited because the sensor is in a fibrous shape.